The use of a tool and fabrication holder mounted in a press is known in sheet metal forming operations. Certain parts of the tool follow the movement of the press slide, while other parts are at rest during a part of the press cycle. One example is where a sheet metal holder is used, which rests on springs, which exert the sheet metal holding force. Other examples of moveable tool parts are shuttles and punches. The springs, at least three in number, are at rest until the upper tool part impacts against the holder and moves this downwards, the springs being tensioned. A direct impact on a stationary sheet metal holder places heavy stresses on the tool and on mechanical parts of the press, whilst unwanted noise and vibrations are generated. An excessive impact between the upper tool part and the sheet metal holder also has a negative effect on the sheet metal forming process itself, since the sheet metal fabrication that is to be formed is situated between upper tool and sheet metal holder, and since there is a risk of the sheet metal lubrication film breaking down, which carries a risk of irregular quality in the subsequent sheet metal drawing process.
After the pressing process, the holder with finish-formed sheet metal part is brought into its original position by the springs returning to their extended standby position. Because a certain return speed is imparted to the holder by the springs, there is a risk that the holder will not stop immediately when the springs reach their limit position, but will continue until the g-force has braked the speed. There is a risk here that both holder and fabrication will lift off entirely from the springs. This “lifting” of the fabrication and the holder causes wear and creates noise. There is also a risk of the holder itself jumping out of its guides.
Attempts have previously been made to solve the aforementioned problem by the use of CNC hydraulic systems or by reducing the impact stresses and the fabrication and holder lifting through the use of separate dampers. The disadvantage of CNC hydraulic systems is the need for a separate hydraulic source and cooling facility for the hydraulic fluid used. Connecting hoses are required. Energy losses occur during acceleration and retardation. Moreover, such a system is relatively complicated and expensive. Damping the impact through the use of separate dampers likewise has disadvantages, partly due to the generation of heat and partly due to the fact that the damping function does not vary according to the pressing speed. Known damping arrangements are often temperature-sensitive and also sensitive to mechanical effects.
A design construction for initiating an downward movement of a fabrication holder is specified in EP 1 034 858 A2. This design construction requires special arrangements for incorporation into a pressing tool and it is doubtful whether the design construction can be retrofitted to existing tools.
As an example of the prior art, reference will also be made to DE 3623188 C1, on the basis of which the pre-characterising clauses of each of the independent claims have been worded.